1. Field of the Invention
The present invention relates to a confined cell structure, and more particularly, to a cell structure with multiple entries guarded by nano-scale metal nano-particles, which help confine subnano-scale metal nano-particles inside the cell, and a method of making the same.
2. Description of the Related Art
Metal nano-particles supported on porous materials have been used extensively in industry. Of particular interest is making highly dispersed metal nano-particles not only to reduce the amount of expensive metal, but also to enhance their reactivity. High dispersion means a large fraction of the metal atoms are located on the particle's surfaces, or at the interface of the support. By being under-coordinated, the surface-, or more specifically the step- and the corner-atoms, are more reactive than the bulk ones.
In the case of subnano-sized particles, where almost every atom or at least one of its closest neighbors is bound to the support through strong metal-support interactions, the chemical nature of the metal particle is strongly dependent on the property of the support. For porous carbon materials, carbon atoms have the unique ability to form chemical bonding states with varied hybridization of 2s and 2p atomic orbitals giving rise to bifunctional catalytic behavior near the junction of the metal-carbon interface. This makes nano-porous carbon materials an ideal support for extremely small metal particles, allowing them to retain or even augment their catalytic activity under catalytic reaction conditions.
However, due to the low stability of the subnano-sized metal particles, their practical use remains difficult. In order to increase the stability, one common method is to form oxygen-containing functional groups on the carbon surface. These oxygen functional groups act as the anchoring sites for the deposited metal particles making the diffusion difficult and thus hindering the ripening process.
Beside reducing the particle size, another method to enhance the reactivity of catalysts is to encapsulate metal nano-particles inside a confined nano-space. This increases the frequency of collisions between the reactants and energetics products enabling the reactions to proceed via some higher energy barrier pathways, which are otherwise inaccessible in an open system, to form products with a global minimum energy state.
To form a confined nano-reactor, Correa-Duarte and co-workers had disclosed in the Angew Chem-Int Edit 51, 3877 (2012) a template method consisting of four steps. The first step is to form spherical templates, made of polystyrene. The second one is to form metal nano-particles on the spherical template. The third one is another deposition procedure to form a layer of porous silicon oxide on the surfaces of the metal-decorated spheres. The last step is to remove the template to create a hollow-sphere with the interior surface decorated with metal nano-particles. Besides the complexity of the procedures, the main disadvantage of above-mentioned template method is the significant modification and the partial, or complete, removal of the metal nano-particles in the template removing process. This inevitable metal removing process significantly modifies the physical and chemical properties of the subnano-sized metal nano-particles making their survival difficult and uncertain. Here we provide a simple method to form a new confined cell structure that consists of multiple entries guarded by nano-scale metal nano-particles, which help confine subnano-sized metal nano-particles inside the cell.